1. Field of the Invention
The present invention relates to a method and apparatus for handling Uplink Shared Channel (UL-SCH) transmission, and more particularly, to a method and apparatus for handling UL-SCH transmission in a UE of a wireless communication system, to prevent an Msg3 transmission from colliding with a retransmission of a previous transmission block (TB) already stored in an UL HARQ buffer of the UE.
2. Description of the Prior Art
Long Term Evolution wireless communications system (LTE system), an advanced high-speed wireless communications system established upon the 3G mobile telecommunications system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B alone rather than in Node B and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In the LTE system, a user equipment (UE) needs to initiate a random access procedure for any of the following events, to connect with a Node B. The events are: (1) Initial access from a RRC_IDLE state; (2) Initial access after a radio link failure; (3) Handover; (4) Downlink data arrival during a RRC_CONNECTED state; (5) Uplink data arrival during RRC_CONNECTED. The random access procedure can be performed by contention-based or non-contention-based manner depending on whether a Random Access Channel (RACH) resource used by the UE is assigned by the network or randomly selected by the UE itself.
Please refer to FIG. 1, which is a schematic diagram of a contention-based random access procedure. As shown in FIG. 1, the contention-based random access procedure mainly includes the following four steps: (1) Step “Random Access Preamble on RACH in uplink”, (2) Step “Random Access Response on Downlink Shared Channel (DL-SCH)”, (3) Step “Scheduled Transmission on Uplink Shared Channel (UL-SCH)”, and (4) Step “Contention Resolution on Physical Downlink Control Channel (PDCCH) or DL-SCH”. First, when RRC layer or MAC layer initiate a random access procedure, a UE randomly selects a RACH resource to transmit a random access preamble, also called Message 1 (Msg1), to Node B for requesting an uplink grant. A Random Access Response message, also called Message 2 (Msg2), carrying an uplink grant and a Temporary Cell Radio Network Temporary Identifier (Temporary C-RNTI) is then transmitted from the network to the UEs those sent the Random Access Preamble. Thus, the UEs using the same Random Access Preamble in Message 1 would receive the same uplink grant and Temporary C-RNTI from Msg2 and use the same uplink grant to transmit a Scheduled Transmission message, also called Message 3 (Msg3), to the Node B, so as to cause contention between the UEs. The content carried by the Msg3 mainly includes uplink data and a User Equipment Identity (UE ID).
According to different trigger events, the UE ID carried in Message 3 can be divided into two types: Cell Radio Network Temporary Identifier MAC control element (C-RNTI MAC CE) and Common Control Channel Service Data Unit (CCCH SDU). The C-RNTI MAC CE includes a C-RNTI of the UE; while the CCCH SDU includes a UE Contention Resolution Identity provided by an upper layer. Therefore, when the Node B outputs a contention resolution message including a specific UE ID, also called Message 4 (Msg4), contention between the UEs can be solved. Please note that the way to handle Msg4 would be different depending on the UE ID type carried in Msg3. As for detailed description of the random access procedure, please refer to related MAC specifications, which are not narrated herein.
On the other hand, a technique of transmission time interval (TTI) bundling is introduced for improving uplink coverage in the prior art. TTI bundling is performed by repeatedly encoding and transmitting a same transport block (TB) in a set of consecutive TTIs, and those repeatedly transmitted packets are named a TTI bundle. UEs in cell boundary utilizing TTI bundling can reduce transmission delay and signaling of control channels for enhancing reliability and accuracy of data transmission, such that LTE uplink coverage can be improved.
According to current specifications, TTI bundling is characterized as below:
(1) The same HARQ process is used for all transmissions within the TTI bundle.
(2) TTI bundling is switched on/off per UE with higher layer signaling, e.g. Radio Resource Control (RRC) signaling. When switched on, TTI bundling would apply to all uplink transmissions using Physical Uplink Shared Channel (PUSCH).
(3) A bundle is treated as a single resource, i.e., a single grant and a single HARQ feedback (e.g. acknowledgement ACK or non-acknowledgement NACK) is used for each bundle.
Therefore, the retransmission of a TTI bundle is also a TTI bundle. However, for each transmission (except a first transmission) within the TTI bundle, non-adaptive retransmissions are generated by the HARQ process according to a size of the TTI bundle, i.e. the number of consecutive TTIs in the bundle, without waiting for HARQ feedback from previous transmissions. Compared to new transmissions and adaptive retransmissions which are performed on the resource indicated on PDCCH, a non-adaptive retransmission is performed on the same resource as was used for the last transmission. Related HARQ operation is known by those skilled in the art, and is not narrated herein.
It is worth noting that the HARQ Round Trip Time (RTT) for TTI bundling is doubled compared with the normal HARQ operation, i.e. non-TTI bundling operation. That is to say, if a first transmission of a bundle occurs at TTI k, retransmission of the bundle starts at TTI (k+2*HARQ_RTT), where HARQ_RTT represents the HARQ RTT of the normal HARQ operation. For example, in the current specification, the normal HARQ RTT is 8 ms, while the HARQ RTT for TTI bundling is 16 ms.
According to the current specification, the HARQ process is applicable for the Msg3 transmission of a random access procedure. However, the network cannot know whether TTI bundling is activated when the UE performs a random access procedure, so TTI bundling does not apply to the Msg3 transmission. Therefore, the HARQ RTT of the Msg3 transmission is the same as the HARQ RTT of the normal HARQ operation even if TTI bundling is active in a UE. In this case, since the UE can only perform one UL HARQ transmission per TTI, the Msg3 transmission may collide with a retransmission of a previous TB already stored in UL HARQ buffer.
For example, please refer to FIG. 2, which illustrates a situation that a UE performs an Msg3 transmission when TTI bundling is activated. According to the current specification, when TTI bundling is activated, the number of HARQ processes is 4, and the size of a TTI bundle is fixed to 4, as shown in FIG. 2. Under this condition, an HARQ RRT for TTI bundling Bundle_RTT is 16 ms. Assume that the UE receives an UL grant for Msg3 transmission at time A and uses a first HARQ process (process id=1) to perform an initial Msg3 transmission. If the first HARQ process is already assigned to a transmission of a TTI bundle, the Msg3 transmission may collide with a non-adaptive retransmission of the TTI bundle.
On the other hand, assume the first HARQ process is not assigned to any TTI bundle transmission and completes the initial Msg3 transmission at time A. If the UE cannot successfully receive an ACK for the Msg3 transmission, the first HARQ process would perform an Msg3 retransmission after the HARQ RTT of the normal HARQ operation, i.e. at time B. In such situation, since the HARQ RTT of the Msg3 transmission is different from the HARQ RTT for TTI bundling, the Msg3 transmission may collide with a non-adaptive retransmission of a TTI bundle performed by a third HARQ process (process id=3).
In fact, even if TTI bundling is not active, the initial Msg3 transmission may also collide with a retransmission of the previous TB already stored in UL HARQ buffer, causing the UE at a loss.